1. Technical Field of the Invention
The present invention relates to the field of Bipolar Junction Transistors (BJT) and, in particular, to a structure and method for reducing current gain variations in a BJT.
2. Background of Invention
Bipolar Junction Transistors are an integral component in modem microelectronics. BJTs have better analog capabilities when compared to CMOS transistors. It is therefore highly desirable to integrate analog BJTs into CMOS processes in order to create CMOS circuits with higher performance abilities. Often, this integration is accomplished with few or no additional fabrication steps relative to the CMOS-only process. However, as CMOS processes extend into the deep sub-micron regime, it is necessary to correspondingly shrink the BJT devices. This reduction in device scale creates a variety of problems.
First, when shrinking the scale of the BJT, the distance between the emitter and the base of the BJT is reduced. This reduction in emitter-base length can lead to increased amounts of Auger recombination at the base. The current gain (beta) of the BJTs is a function of the base current. Therefore, increased amounts of Auger recombination at the base causes a reduction in the current gain.
Second, the shrinking of BJT dimensions requires that well profiles are tailored using multiple chain implants instead of long high-temperature diffusion steps. These chain implants create significant numbers of interface and bulk traps between the emitter and the base contact that facilitate recombination. This high density of interface traps, also known as surface states, within the BJT do not get annealed during subsequent processing because of low thermal budget processing. Otherwise, the whole purpose of using chain implants is defeated. These surface states lead to an increased base recombination current in the BJT. The surface state density is unpredictable, not well controlled, and is known to vary across a wafer, lot-to-lot, and facility-to-facility. Since the base current is dominated by recombination due to the presence of a high surface state density, the current gain of these devices is found to vary significantly even across a single wafer. This current gain variation, especially for analog circuits, is very undesirable. It is therefore highly desirable to develop a structure that can reduce this variation in the BJT current gain as BJTs are integrated in deep sub-micron processes.
There currently are known techniques that address this current gain variation. One such technique is through using a field oxide between the emitter and base. Through having a field oxide between the emitter and base, it is possible to significantly reduce the flow of electrons from the emitter into the base where they would recombine. In addition, the field oxide reduces the number of interface traps encountered by the injected electrons. However, this technique has its scale limits.
One other method of addressing this current gain problem is through using Shallow Trench Isolation (STI) to separate the emitter and base. This method is a natural evolution from the use of the field oxide. Just as with the field oxide, the STI significantly reduces the flow of electrons from the emitter to the base, thereby reducing recombination from Auger processes and interface traps.
In addition, both the field oxide and STI solutions create new problems. In both field oxide and STI designs, the isolation between the base and emitter extends all the way to the edge of the emitter. Both the field oxide and the STI block a sidewall of the emitter. Consequently, the field oxide and STI reduce the surface area of the emitter that can inject electrons into the base. As a result, fewer electrons are injected into the base and the drive current capabilities of the BJT are reduced.
Another method of reducing the current gain problems created by device shrinkage is disclosed in T. Terashima et al, Multi-Voltage Device Integration Technique for 0.5 xcexcm BiCMOS and DMOS Process, Proceedings of the ISPSD 2000 Conference, Toulouse, France, pp. 331-334, (May 2000). Terashima et al discloses an across-wafer current gain variation of 50 to 95 for an NPN BJT. The cause of this current gain variation is attributed to a high interface state density that did not get fully annealed. The disclosed solution was to implement a p+ base ring completely surrounding the emitter with a set spacing between the emitter n+ region and base p+ ring. This solution reduces current gain variation only for a spacing of 0.5 xcexcm or less. However, this solution still has a fairly high current gain variation, approximately 70 to 90, across the wafer. This current gain variation presents problems as devices are reduced in scale.
Despite the above solutions, there is a continuing need to develop new structures to reduce the current gain variation in BJTs as devices are integrated into sub-micron processes.
A preferred embodiment of the present invention utilizes a trench pullback positioned between the emitter and base to reduce the current gain variation. This structure reduces the current gain variation by limiting recombination. In addition, this structure takes advantage of the lateral component of the emitter current, allowing it to contribute to the current gain of the device. More specifically, the trench pullback is comprised of a trench and an active region. The trench is positioned next to the base and extends partially toward the emitter. The trench in effect is pulled back from the emitter leaving an active region. The area between the emitter and the trench is the active region.
There are two primary forms of recombination that occur between the base contact and the emitter in the BJT. The first is Auger recombination that occurs at the base contact due to the lateral electron current injected by the emitter. The second is Shockley-Read-Hall (SRH) recombination that occurs at interface traps formed at the surface of the BJT between the base and the emitter. There is also SRH recombination in the bulk. However, the SRH recombination at the surface states is the dominant process. As the base injects holes into the emitter region and the emitter injects electrons into the base region, these interface states facilitate recombination.
The shrinkage of BJT dimensions into the sub-micron region exacerbates these recombination problems. Bringing the base and the emitter closer together increases the amount of Auger recombination. Sub-micron processes prevent the use of thermal annealing that reduces surface and bulk states. Further, sub-micron processes require the use of chain implants that generate significant numbers of surface states. The trench pullback of the present invention addresses these problems.
First, the presence of the trench increases the physical distance that electrons injected from the emitter must travel to reach the base contact. This increase in distance thereby reduces the amount of Auger recombination. In addition, the presence of the trench reduces the interaction of the injected electrons with the interface states. The trench occupies much of the space between the base and the emitter thereby reducing the number of surface states that these injected electrons encounter. The remaining surface states that the injected electrons are exposed to are in the active region. The trench structure has further benefits that reduce the amount of recombination that occurs at these remaining interface states in the active region. The trench also increases the distance that holes injected by the base must travel in order to reach these interface traps. Recombination at these interface states occurs when electrons and holes reach the trap. Through increasing the distance that the holes must travel to reach these interface traps, the trench reduces the occurrence of recombination at the remaining interface states in the active region.
Through reducing the recombination due to Auger processes and interface traps, the base current is reduced. Correspondingly, the current gain of the BJT is increased. In addition, because the effect of the recombination at the surface states is reduced, it is possible to fabricate BJTs with a specified level of gain reliably in a reproducible manner.